A variety of peptides which have been localized previously to nerve and gut tissues have also been found in the retina, primarily in amacrine cells. The retina is uniquely well suited for study of these neuropeptides because its anatomy, physiology and pharmacology have been characterized extensively, and it can be activated by the same light stimuli in vitro that it responds to in vivo. By further examination of retinal peptides, specifically somatostatin, this proposal aims 1) to elucidate the neurotransmitter function of neuropeptides, 2) to determine the functional significance of retinal amacrine cells, and 3) to advance current knowledge regarding the biochemistry of somatostatin. Anatomic localization of somatostatin-like immunoreactivity (SLI) in the frog retina will be examined. Using light and electromicroscopic immunohistochemical techniques, the relationship of SLI-containing cells to other retinal cells, especially those containing neuropeptides will be studied. Retinal SLI has been shown inprevious studies to appear in multiple molecular forms. Both large and small SLI will be purified from bovine retinas and whole brains and from fffrog brains. The bioactivity of these purified peptides will be assayed by measuring their effect on growth hormone release from CH4C1 pituitary tumor cells. Receptor binding characteristics of the isolated peptides will be tested in both GH$C1 cells and frog retinas. Biosynthesis and release of SLI will be examined in pulse-chase experiments using 34S-cysteine. Immuncprecipitated radioactivity will be analyzed by gel filtration, gel electrophoresis, and high pressure liquid chromatography. Cell-free synthesis lf SLI will be attempted in a reticulocyte lysate system using mRNA purified from frog retinas. Regulation of frog retinal SLI content in vivo and SLI release from cultured frog retinas in vitro will be examined using various light and chemical stimuli. Similar experiments will be attempted with cells isolated and enriched from frog retinas by counterflow elutriation and d ensity gradient centrifugation.